Meninges, Ventricular System, & Blood Supply of the Brain

What are the meninges?

Continuous 3-layered membrane covering of brain & spinal cord that spans from cranial vault to S2

What are the functions of the meninges?

• Maintain shape & position of brain & spinal cord

• Mechanical suspension

• Buoyancy effect from CSF

• Isolate CNS extracellular fluid from body’s extracellular fluid

What is the dura mater?

• Thick outer layer

• Vascular 

• Well-innervated (pain)

What is the arachnoid mater?

• Thin, delicate middle layer with web-like appearance 

• Avascular 

What is the pia mater?

• Very thin & delicate inner layer 

• Tightly stuck to cortex

• Highly vascular 

What structures make up the dura mater?

• Periosteal layer → tightly fused to skull

• Meningeal layer → compressed b/w periosteal dura & arachnoid mater

What are the dural septa?

• Infoldings of meningeal dura within cranial vault

• Provide buoyancy, protection, suspension

• Form intracranial compartments

What is the falx cerebri?

• Dural septa between 2 cerebral hemispheres

• Runs down the middle 

What is the tentorium cerebelli?

• Dural septa between hemispheres & cerebellum

• Lays like a tent over cerebellum

What compartments are created by the tentorium cerebelli?

• Supratentorial compartment → contains 2 hemispheres

• Infratentorial compartment → contains cerebellum & brainstem

What is the tentorial notch?

Midline opening at the anterior edge of tentorium cerebelli where the midbrain passes through

What are the dural sinuses?

• Endothelium-lined cavities between periosteal & meningeal layers of dura

• Cerebral veins drain into sinuses

What is the flow of blood through the dural sinuses?

• Brain veins

• Superior sagittal sinus or Strait sinus

• Confluence of sinuses

• Transverse sinuses

• Sigmoid sinus

• Internal jugular vein

• Heart

What is the cranial epidural space?

Potential space between skull and periosteal layer of dura 

What is the cranial subdural space?

Potential space between dura & arachnoid

Define potential space.

• Not present under normal conditions

• Fluid can accumulate here if pathology/trauma/injury occurs

What is the blood supply to the cranial dura?

• Primary middle meningeal artery

• Travels in epidural space

What causes the cranial dura to be pain sensitive?

• Somatic sensory nociceptors

• Found in: CN V Trigeminal, CN X Vagus, & C2-C3 cervical nerves

• Nociceptors stimulated by mechanical distortion of dura & chemoreceptors

What causes forehead/eye/temple headaches?

CN V, Trigeminal nerve 

What causes posterior head/neck & behind the ears headaches?

CN X, Vagus nerve & C2-C3

How does the arachnoid mater receive nutrition if it is avascular?

Through diffusion from dura

What is the cranial subarachnoid space?

• Real space between arachnoid & pia layers

• Continuous through the spinal cord & cranium

• Filled with CSF

• Also contains nerves & vascular structures

Define subarachnoid cisterns.

• Larger spaces where it bridges over larger surface irregularities 

• Contains lots of CSF

What are arachnoid trabeculae?

• Flattened, irregularly shaped fibroblasts

• Connect arachnoid to pia mater

• Provide mechanical support for CNS

What are arachnoid villi?

• Small protrusions of arachnoid mater through dura & into venous sinuses

• Sites for CSF reabsorption

• Where CSF comes into contact with venous blood

Define arachnoid granulations.

• Clusters of arachnoid villi into dural venous sinuses 

• Allow CSF to move one way into the blood, not back (like valves)

• Mainly in superior sagittal sinus region

What is the cranial pia mater?

• Vascular membrane

• Completely stuck to the brain

• Follows blood vessels through the brain

What is the spinal cord dura?

Continuation of cranial dura

What is the spinal cord epidural space?

• Real space between dura & vertebral periosteum

• Contains fat, connective tissue, vascular structures, nerve roots

• Used for anesthesia to numb nerves

What is the dural sac?

• Sac of CSF around the spinal cord

• Extends from foramen magnum to S2

• Contains spinal cord & cauda equina  

What is the spinal cord subarachnoid space?

• Real space between arachnoid & pia 

• Contains vascular structures, nerve roots, & CSF

• Anesthesia at L4-L5 paralyzes waist down

Define lumbar cistern.

• Area of enlarged subarachnoid space

• Contains cauda equina

• L1/L2 to S2

• Area for spinal taps & anesthesia

What is an epidural hematoma?

• Tearing of meningeal arteries

• Bleeding between skull and dura mater

• Fast bleeding (arterial)

• Medical emergency STAT

What is a subdural hematoma?

• Tearing of veins where they enter venous sinuses 

• Bleeding between dura & arachnoid

• Slow bleeder (venous)

What are herniations?

• Brain tissue pushed from one compartment to the other

• Due to increased ICP from hemorrhage, CSF build-up, or inflammation

What is a subfalcine herniation?

Cingulate gyrus slips under falx cerebri

What is an uncal herniation?

Medial temporal lobe herniates through tentorial notch and presses against the midbrain

What is a tonsillar herniation?

• Cerebellar mass causes tonsil to herniate through foramen magnum & compresses the medulla

• Common in children

What are ventricles?

• Cavities that produce & circulate CSF throughout the CNS

• Lined with ependymal cells to produce CSF

What are the lateral ventricles?

• 2 C-shaped cavities in each hemisphere 

• Communicates with 3rd ventricle through interventricular foramina (2)

What is the third ventricle?

• Midline cavity of diencephalon

• Found between left and right thalamic & hypothalamic bodies

• Communicates with 4th ventricle through cerebral aqueduct in midbrain

What is the 4th ventricle?

• Midline cavity between brainstem & cerebellum

• Communicates with subarachnoid space through median aperture & lateral apertures

What is the central canal?

• Narrow midline cavity along the whole length of the spinal cord

• Continuation of the 4th ventricle into the spinal cord

• How the spinal cord gets CSF

What is the choroid plexus?

• Specialized strands of epithelium found in all ventricles but mostly lateral ventricles

• Filters arterial blood to create CSF to sent to the ventricles

Define cerebrospinal fluid (CSF).

• Clear, colorless liquid low in cells & proteins 

• Produced/secreted by choroid plexus

• About 150 mL in CNS

• Delivers nutrients, clears waste, protects the CNS

What is the CSF circulation pathway?

• CSF produced/secreted by choroid plexus into ventricles

• Lateral ventricles to 3rd ventricle via interventricular foramina (2)

• From 3rd ventricle to 4th ventricle via cerebral aqueduct

• Exits 4th ventricle via median aperture to cisterna magna & from lateral apertures to pontine cistern

• CSF released into subarachnoid space

• CSF reabsorbed into venous system through arachnoid villi

• Ends in superior sagittal sinus 

What is hydrocephalus?

• Disruption of CSF circulation due to increased CSF pressure

• Excess production of CSF, blockage of CSF circulation, or deficiency in CSF reabsorption

What are signs of hydrocephalus in an infant?

• Abnormally large head

• Bulging fontanelle

• Setting sun eye sign

• Seizures

• Sleepiness

• Vomiting

• Irritability 

What are potential causes of hydrocephalus?

• Congenital malformations

• Meningitis

• Brain Injury

• Myelomeningocele (spina bifida)

What are the 2 main treatments for hydrocephalus?

• Ventriculo-atrial (VA) shunt → drains CSF from ventricle to right atrium

• Ventriculo-peritoneal (VP) shunt → drains CSF from ventricle to peritoneal cavity

  • MOST COMMON

What is the function of the blood-brain barrier?

Allows selected substances to enter the blood of the brain

What is the true blood-brain barrier?

• Relies on tight junctions b/w the endothelial cells of tunica intima of all cerebral vessels 

• Forms extracellular CNS fluid (about 65% glucose)

What is the blood-CSF barrier?

• Between vasculature of the choroid plexus & ventricles

• Tight junctions of choroid epithelial cells of choroid plexus regulate which substances can enter the ventricles part of CSF

Where do superficial cerebral veins empty into?

Superior sagittal sinus

Where do deep cerebral veins empty into?

Strait sinus 

What is the confluence of sinuses?

Where superior sagittal sinus, strait sinus, & transverse sinuses meet

Where do the transverse sinuses drain into?

Sigmoid sinus

Where do sigmoid sinuses drain into?

Internal jugular veins